Maybe add a count of the number of other columns which currently use the trigger? I guess you'd have to do a policy-violating query to include the comfort-inducing "no other columns use this trigger" text, but I think that's pretty ok? Or inversely, just a warning "X other columns are currently using this trigger".

NB: I assumed this code was invoking destroyObjectPermanently; disregard.

Just out of curiosity, what's the scope on these constants? Do they need to be globally unique? Would we ever know if we reused one by accident? In practice I guess we scope transactions per-application since they have their own DBs anyway.

Possibly we should count and give the user an option to destroy it (or, more likely, banish it to the shadow realm). Elsewhere, I'm policy-violating on count/existence anyway to provide users a hint before they edit a trigger in use on a lot of boards they can't see, so I think this is fine from a policy perspective. It's generally okay to know that objects exist and what their PHIDs are, just rarely much more than that.

(For example, you can generally see all the edges attached to an object, even if D123 is attached to T123 which you can't see.)

Right, this is just so that when you bin/remove destroy <phid> a trigger we end up with minimally-weird column state since I don't really want to write all the UI for "this column references a trigger which no longer exists" and doubt the outcome would ever be any different from removing it, practically.

Some of the older stuff namespaces them heavily, but in practice all the transaction stuff is sharded into per-object tables anyway and there's no transaction.search mode which says "give me every subscription change to every object ever", nor is there likely to ever be, I think. I currently believe the handful of cases for this are probably better served by fact extraction in the Facts ETL pipeline, so we don't have to run 250 queries against every single whatever.transaction table.